The replacement of fossil energies in order to ensure future energy needs constitutes an essential preoccupation. In this context, biodiesel constitutes a substitute for fuels produced using petroleum (petrodiesel). The term “biodiesel” should be understood to mean fuels that use, as raw material, natural products resulting from cultivation or farming, i.e. renewable products.
This approach, which has been used for several decades, has been the subject of considerable studies which have resulted in biofuels for diesel engines being placed on the market. The basis of these studies has been the use of vegetable oils obtained from seeds of various plants, such as rape, soybean, sunflower, palm, etc., or of animal fats which consist of a mixture of triglycerides of fatty organic acids, the chain length of which is generally between 16 and 18 carbon atoms. Thus, a range of biodiesels has been developed, known under the abbreviation FAME for Fatty Acid. Methyl Ester, a predominant molecule of which is the methyl ester of oleic acid. Similarly, EASE (Fatty Acid Ethyl. Ester) has been developed. They form what are known as first-generation biodiesels.
These esters can be obtained by direct transesterification of the vegetable oil, obtained from the seeds of these plants, and by analogy transesterification of the anima fat, in the presence of methanol, which results in a mixture of esters of which the formulae depend on the nature of the oils or fats used as feedstock.
These esters can also be obtained, in a first step, by hydrolysis of the triglycerides contained in these oils or fats, and, then, in a second step, by esterification with methanol. Between these two steps, it is possible to perform a separation of the acids in order to obtain an ester mixture enriched in at least one of these esters.
The fatty acid compositions of a vast majority of oils and fats can be found in the publication “Pure and Applied Chemistry 73, 685-744” IUPAC 2001, this list having been established as it happens for food use, but which can in all, cases be used for the production of biodiesels. Aside from a few “exotic” examples such as coconut oil, palm kernel oil and babassu oil which contain “short” fatty acids with 12 carbon atoms, all the others are based essentially on saturated or unsaturated C16 and C18 acids.
Moreover, a broad study carried out by Gerhard Knothe and his collaborators has analyzed the key factors in the choices made in terms of fatty esters for the formulation of biodiesels. This study is summarized in the article entitled. “Designer” Biodiesel: Optimizing Fatty Ester Composition to Improve Fuels Properties published in “Energy & Fuels” 2008, 22, 1358-1364. This article deals with the influence of the structure of the fatty acids, namely their chain length, the presence or absence of double bonds and the presence of hydroxyl functions, on their properties from the viewpoint of the specifications for diesel fuel as defined in standards ASTM D6751 and EN 14214, and in particular the cetane number, the viscosity, the cold flow properties and the oxidation stability. The conclusion of this study is that methyl oleate is indeed the main basic molecule of the mixture, the performance levels of which can be improved by adding other specific esters.
This study is essentially technical. From a practical point of view, it is also necessary to look at this from a political point of view, taking into account an important additional parameter, namely that this crop product or the cultivation of this product should not have an effect on the development of the crop intended for human food that is essential with the change in the world population. It is becoming increasingly important to develop industrial crops which are not in competition with the food applications.
To summarize, the selection of one of these routes naturally depends on the “thermal and energy” properties of the fuels thus obtained, but also on three important additional criteria, namely compatibility of these fuels with the engines currently used, compliance with the new standards for CO2 emissions and harmonization (noncompetition) with the crops dedicated to food.
“Second-generation” biodiesels are also known, which are obtained by hydrotreating the vegetable oils, resulting, by hydrogenation, in isomerized or nonisomerized long-chain hydrocarbons. The isomerization of the paraffins makes it possible to significantly reduce the cloud point, i.e. the temperature at which the paraffins begin to crystallize.
Finally, it has also been proposed, in an article by N. M. Irving published in the 16th European Biomass Conference, Jun. 2-6, 2008, Valencia, Spain, under the title “Clean, High Enthalpy Biofuels”, to replace, by nitrilation, the acid function of the fatty acid with a nitrile function. These fatty nitriles, obtained either by nitrilation of the natural fatty acids obtained by hydrolysis of the triglycerides of the oil resulting from the seeds, said acids comprising chains of between C12 and C18 and centered mainly on C16, or by direct nitrilation of the oil, appear to give excellent results as biodiesels.
The problem to be solved is therefore that of finding a diesel fuel based on a renewable source, which as much as possible meets the specifications for biodiesels (standards ASTM P6751 and EN 14214) and in particular the octane number, melting point, viscosity and oxidation stability criteria, using a source which does not lead to the production of food products in order to avoid any “prohibited” competition. These biodiesels must of course be completely compatible with the fuels from oil with which they are, so far, most of the time used as a mixture.
Among these possible sources, mention may be made of oils containing hydroxy acids which are not very suitable for human food, and in particular the castor plant. Indeed, the castor seed has an oil content of approximately 50%. In addition, castor oil, which contains more than 80% by weight of ricinoleic acid and approximately 15% by weight of oleic acid and linoleic acid, has no food application, which is of considerable advantage in the current fuel versus food debate. This oil therefore appeared to be able to constitute an excellent source for the production of biodiesels, all, the more so since this plant has a particularly high yield per hectare and can also grow in very difficult soils and under conditions with low rainfall, where few food plants can be cultivated, which limits, a fortiori, the competition with plants for food applications. Unfortunately, as reported by G. Knothe on page 1364 of the article “Energy & Fuels” mentioned above, methyl ricinoleate has properties which a priori exclude its use as a base in a biodiesel application. Specifically, its viscosity at low temperature is very high, its melting point is close to 0° C. and its cetane number is well below the specification. To complete the picture, it may be added that its oxidation stability is lower than those of methyl oleate and methyl linoleate.
The use of castor oil in the biodiesel application is also mentioned in the article “Thermoanalytical characterization, of castor oil biodiesel” by Marta M. Conceicao at al, published in Renewable and Sustainable Energy Reviews—11 (2007) 964-975 (Elsevier). This essentially analytical, article illustrates the problem of the viscosity and also refers to a very high density which in fact is outside diesel specifications. The octane aspect is addressed only incidentally on page 969, paragraph 2, indicating that transesterification reduces the viscosity of the oil without modifying the octane thereof, the value of which is not known. The unsuitability of the castor plant for this application is probably due to the presence of the OH radical in the β-position with respect to the double bond. The same problem is found with the seeds of plants of the Lesquerella genus, in particular Lesquerella fendieri, from which is extracted an it containing lesofuerolic acid at more than 50% by weight and, as a mixture, approximately 35% by weight of various, in the majority unsaturated, C18 acids.
The obvious advantage of castor oil as a base for a biodiesel has led some researchers, as reported by G. Knothe, to carry out studies on the genetic modification of the castor seed species with a view to producing much more oleic acid and less ricinoleic acid. In this respect, mention may be made of the studies by Pilar Rolas-Barros published in Crop Science, vol. 44, January-February 2004, pp. 76-80 and vol. 45, January-February 2005, pp. 157-162.